The color eliminating circuit in a color television system is a circuit in which a color burst signal, in a chromatic signal (color signal) derived from a video signal, is detected by a killer detection circuit. Whether the broadcast is a color broadcast or a black-and-white broadcast is discriminated on the basis of the presence or absence of the burst signal, and the amplification function of a band amplifier circuit for amplifying the chromatic signal is interrupted for a black-and-white broadcast free of the burst signal, to prevent any unnatural color from being presented in a black-and-white picture.
In general, the receiving apparatus includes an automatic gain control circuit, and hence, where the field strength of received signals is low, the gain of an input signal is significantly increased, with the result that the amount of noise inevitably increases. Also, in a color television system, where the field strength of received signals has dropped to a low level, demodulation and reproduction of faithful colors are impossible because of the presence of noise, etc., and it becomes difficult to see the color picture. The color eliminating circuit is, therefore, operated so that the amplification function of the band amplifier circuit for amplifying the chromatic signal is interrupted for a color broadcast as in black-and-white broadcasts.
The killer detection output voltage (V.sub.K) - versus-field strength of a received electric wave (E.sub.i) characteristic of the killer detection circuit employed in present day color eliminating circuits is shown in broken lines l.sub.o ' in FIG. 1.
More specifically, the killer detection output V.sub.K is the sum of a component l.sub.1 (the one-dot chain line in FIG. 1) obtained by detecting the burst signal in the chromatic signal and a component l.sub.2 (the two-dot chain line in FIG. 1) obtained by detecting the noise signal in the chromatic signal. As the field strength E.sub.i of the received signals decreases, the burst component l.sub.1 decreases, whereas the noise component l.sub.2 increases. Consequently, the killer detection output V.sub.k decreases with the lowering of the field strength E.sub.i of received signals, but when the field strength E.sub.i of the received signals becomes extremely low, the noise component l.sub.2 becomes larger than the burst component l.sub.1 and the killer detection output V.sub.k increases again.
On the other hand, the band amplifier circuit for amplifying the chromatic signal has its gain controlled by the killer detection output V.sub.k, namely the killer voltage, and effects a color elimination operation. At this time, the band amplifier circuit has a predetermined operating voltage V.sub.ko relative to the killer voltage V.sub.k ; it has a high gain and effects chromatic signal amplification upon the application of a killer voltage higher than the operating voltage, and it has a low gain and interrupts the chromatic signal amplification upon the application of a killer voltage lower than the operating voltage, thus effecting a color eliminating operation.
The operating voltage V.sub.ko, at which the color eliminating operation is effected, is generally set at a low level (the solid line l.sub.3 in FIG. 1) and, as previously explained, the amplification function of the band amplifier circuit is interrupted for a black-and-white broadcast; where the field strength of received signals has become low, it is similarly interrupted.
With such a color eliminating circuit, however, where the field strength E.sub.i of the received signals has become extremely low, there is a phenomenon in which the killer voltage exceeds the aforesaid operating voltage V.sub.ko due to an increase in the noise component in the chromatic signal, the gain of the band amplifier circuit becomes large, and color noise is present in a picture. Such a phenomenon also occurs where an idle channel, absent a broadcast signal, is selected.